The invention relates to a viscous coupling for particular use as a brake and comprises a housing and a hub arranged coaxially around one another. Both are rotatable relative to one another and relative to an axis of rotation which also forms a longitudinal axis of the viscous coupling. The housing and hub enclose an inner chamber containing two sets of annular plates. One set forms outer plates, and the other set forms inner plates. The inner plates are arranged along the longitudinal axis to alternate relative to the outer plates in a certain sequence. The outer and inner plates partially overlap one another in the radial direction. The part of the inner chamber which is not occupied by plates is at least partially filled with a highly viscous medium, and preferably silicone oil.
A viscous coupling is described in U.S. Pat. No. 4,889,206. One of the two sets of plates comprises first plates which are non-rotatably connected to the associated coupling part (housing or hub). The second plates of the same set, along a predetermined rotational path, are able to carry out a relative movement relative to the coupling part and relative to the first plates of the set of plates. The relative rotation leads to an axial adjustment which changes the gap between the plates. Thus, as a function of the direction of rotation, different torque characteristics are achieved. The viscous coupling is intended for use in the driveline of the selectively driven rear wheels of a motor vehicle having a permanent drive for the front wheels.
A viscous coupling used as a brake in a driving device for vertically movable doors is described in DE 44 36 237 C1. Firebreak doors are automatically movable from a raised open position to a closed position and are provided in the form of roll-up doors. The doors must move automatically into the closed position when a fire breaks out. And, the door must move into the closed position by its own weight. This movement must also be delayed. However, with roll-up doors it is important for initial acceleration to be possible; but, the closed position be assumed under slowed-down conditions to prevent any damage to the door.
It is therefore the object of the invention to propose a viscous coupling which can also be used as a brake whose driving and braking torque curves increase in steps.